At the present time, air rotary drilling can be conducted in loose unconsolidated sediments when a dual rotary drilling system, such as a Barber DR 26/12 drill is utilized. However, the method is severely hindered by uncontrollable formation heaves that occur when the air circulation ceases.
According to known practices, lengths of drill rod and casing are added and connected to the well bore by threads for the rods and a welded joint for the drill casing. Each of the drill rod and casing are positioned so that they are attached beneath a discharge snorkel. The rod and casing lengths are typically either 10 or 20 feet.
To begin drilling, compressed air is piped through the top drive to the interior of the drill rod. As the pressure builds, the air overcomes the hydrostatic pressure in the casing, escapes out of the drill bit, and causes any water or material in the annular space between the drill rod and the casing to move upward toward the surface. The air flow circuit is completed as the material reaches the discharge snorkel and hose where it is discharged to the ground.
As air circulation becomes established the sediments are loosened by both the drill bit and escaping air. In a manner, similar to an air lift pumping system, the sediments and formation water are carried to the surface by the air. The resulting excavation at the bottom of the drill pipe enables advancement of the pipe. This same pipe rotates during the drilling to minimize skin friction.
Upon reaching the depth to where it becomes necessary to add the next length of drill rod and casing, the introduction of compressed air is stopped. It is at this instant that the heaving begins to occur. Since the drill rod and annular space are occupied only by the remnants of the air, they are essentially at atmospheric pressure. Meanwhile, the formation is under hydrostatic pressure equivalent to the weight of the overlying water. Thus, a very large pressure differential exists at the bottom of the drill pipe. In response to this differential, water rushes up through the annular space, with a velocity which is high enough to keep sediments as coarse as medium sand in suspension. As the water level in the annular space reaches the same level as that in the formation, the water velocity in the annular space drops to 0 and all of the sand in suspension settles out to the bottom of the hole.
The displaced and settled sand phenomena referred to as heave can be a significant problem. The amount of heave that occurs is dependent upon the depth beneath the water table, and the grain size distribution of the formation encountered by the drill bit. Sand heaves on the order of 250-300 feet have been encountered. The sand heave smothers the drill bit and does not permit the reestablishment of air circulation.
As a result of heave, it is necessary to retract the drill rod to a point where the drill bit is above the sand heave. The interior of the drill rod is then flushed by water jetting to make sure it is clean and free the entire length to the drill bit.
The sand is then removed from the inside of the drill pipe by circulating water through the drill system, and flushing the sand up through the annular space to the surface. Since the well is full of water, no additional heave occurs at this point. The flushing process continues, until the drill bit is again situated at the bottom of the drill pipe. The pullback and flushing operation typically takes between 4-6 hours.